Add test case in test-defsystem-depends-on-phase-overlap.script for a failure
observed when running (test-system "cl-protobufs"):
the action (test-op "cl-protobufs") depends on (load-op "cl-protobufs"),
which gets staged at level 0; it then depends on (test-op "cl-protobufs-tests")
which defsystem-depends-on (load-op "cl-protobufs"), but it has already been
staged, so traverse-action fails to re-stage it at higher priority for level 1,
and fails to perform it before we're back to defsystem "cl-protobufs-tests".